Lower Sequence Boundary Research Articles

Facies analysis, sedimentary environments and correlative sequence stratigraphy of Gachsaran formation in SW Iran

Lower Miocene in SW Iran consists of Gachsaran formation. This formation forms one of the main caprocks of reservoirs of oil fields in the SW of Iran that comprises a thick succession of evaporitic rocks in the Zagros basin. This study presents a facies analysis, sedimentary environments and correlative sequence stratigraphy in the SW of Iran. To study petrography, facies, sedimentary environments and sequence stratigraphy of Gachsaran formation, 10 subsurface sections from Aghajari and Rag-e-Sefid oil fields in the SW of Iran and one surface stratigraphic section in the Shiraz area have been chosen and studied (Hossini-Al-Hashemi section). Facies analysis led to the recognition of 13 facies which are deposited in 2 major sub sedimentary environments, tidal flat (sabkha) and lagoon in homoclinal ramp sedimentary environment. Petrography, microfacies analysis and sedimentary environment of Gachsaran formation show this formation in the study area consists of 4 depositional sequences. These depositional sequences have been separated by both type-1 and type-2 sequence boundaries. Based on gamma-ray, sonic log and thin section from cutting sample permit the identification of six 5th, two 4th and a small part of 3rd order depositional sequences in caprock member 1 and lower part of member 2 Gachsaran formation. Lower sequence boundary with Asmari formation is erosional SB1. Fifth order sequences are observed in a different thickness between 4 and 10 m. In caprock two 4th order sequence with the thickness of about 20 m was recognized, which is only a part of the depositional sequence (3rd order). Lower boundary with Asmari formation is SB1. TST systems tract consists of sabkha and lagoon facies parasequences. MFS in this sequence are designated by bitominous shale and early HST systems tract consists of interbedded sabkha and lagoon facies. Gachsaran Formation in the surface section consists of nine 4th order depositional sequences, which are located in four 3rd order depositional sequences. These parasequences are composed of evaparite carbonate succession of green marl, limestone, sandstone, gypsum and red marl, respectively. In Oligocene–Miocene sea-level transgressive and carbonate Asmari formation has deposited in the south-west of Iran and in the Miocene sea-level regressive, and Gachsaran formation has deposited in the study area under a succession of sabkha/tidal flat conditions.

Lombardi reservoir, San Ardo field, California: Sequence stratigraphy in a deltaic environment associated with local tectoni

The Lombardi reservoir at San Ardo oil field in California is extremely sand rich (>90%) and appears to be a relatively simple reservoir several hundred feet thick. The purpose of this study is to characterize stratigraphic complexities in the Lombardi reservoir that may affect later production and interpret how changes in stratigraphic architecture are tied to local syndepositional tectonism. Two parasequence sets are present in the Lombardi: the reservoir exhibits clinoform geometries along the western edge of the field and stacking patterns within the reservoir change across the field. Based on stacking pattern, core, and correlation studies, two possible sequence boundaries are recognized in the Lombardi sand. A lower sequence boundary is proposed approximately in the middle of the Lombardi sand, and an upper sequence boundary is proposed at the top of the Lombardi sand. In the southern part of the field, the Lombardi sand consists of two progradational parasequence sets, separated by the mid-Lombardi sequence boundary. The upper sequence boundary clearly truncates underlying folded Lombardi sand. Just a few miles (∼5 km) away, in the northwest part of the field, the Lombardi sand consists of a lower progradational parasequence set and an upper retrogradational parasequence set, separated by the mid-Lombardi sequence boundary. The upper sequence boundary at the top of the Lombardi sand shows little evidence of truncation of underlying units and is recognized by onlapping of younger strata. The difference in reservoir architecture between the south and northwest part of the field, a distance of only a few miles (∼5 km), is interpreted to be related to syndepositional uplift accompanying tectonism in the south. In core, the Lombardi sand consists mostly of homogeneous sand and bioturbated sand, arranged in upward-coarsening successions. The Lombardi sand is interpreted to be the product of deltaic deposition based on (1) the presence of homogeneous and laminated sandstones interpreted to represent mass flow deposits and (2) its restricted geographic distribution within the Salinas basin. No evidence of deposition in a wave-dominated or tidal-dominated shoreline is indicated. The interpretation presented here is that the sediments were deposited on the front of a delta emptying into a wave-protected estuary or bay, with minimal tidal effects. Complexities in stratigraphic architecture along the western edge of the field do not currently affect production but will likely become important during later stages of field development. That is, in the developed part of the field, reservoir continuity is not an issue, because of high sand content and sheet-like geometries. However, at the margins of the field, which will be produced in the future, clinoform geometries and sandstone tongues clearly affect reservoir continuity and will create vertical compartments during draining.

Stochastic 2D Well-Path Assessments for Naturally Fractured Carbonate Reservoirs

Summary Simple but geologically reasonable and calibrated 2D stochastic models are useful to quantify significant risks and uncertainties associated with alternative-development-well trajectories, particularly when statistical relationships can be established to help quantify those risks and uncertainties, and when the geologic features that create the risks and uncertainties are not adequately addressed within reservoir-flow models. Our example stochastic 2D model considered the naturally fractured depositional-slope region of an isolated carbonate buildup, and the model was populated with relevant features including distributions and geometric details of natural fractures, natural-fracture clustering, and intraformational slope clinoforms that define a mechanically layered sequence. The model was calibrated by use of well-production results and production-logging data so that it reproduced observed well results for cases where the lower sequence boundary does not occur above the oil/water contact (OWC), adding confidence that the model could be used to represent the statistical effect of various alternative trajectories for future wells. Experimental design (ED) was used to determine the significant uncertainties and well-path decisions. Heel and toe elevation and the number of clinoforms encountered by the well were the only significant variables for modeling the frequency of water production. For modeling the frequency of direct well communication to the gas cap, the same variables were significant, in addition to well direction, completion length, and fracture density. The amount of fracture clustering applied in the model was also significant. For our example case, changing the well-elevation profile was effective in managing gas or water risks; however, tradeoffs were evident—and quantified—in attempting to simultaneously address both risks. Minimizing drawdown was not an effective strategy because productivity was low and rarely resulted in economic water-free production if any open fracture connected the well with the aquifer.

Sedimentologic Observation and Stratigraphic Interpretation of the Lower Devonian (Lochkovian) Manlius Formation along the Mohawk River Valley in Upstate New York

Observations from nine measured sections through the lower Helderberg Group across upstate New York suggest that component rock units within the Manlius Formation extend laterally over 150 km across upstate New York, the most notable being a meter‐thick zone of microbial bioherms at the base of the formation. Overlying this zone of microbial bioherms is a 5–10‐m‐thick zone of wavy‐ and lenticular‐bedded ribbon rocks that grade up into microbial‐laminated mudstones that are, in turn, overlain by disrupted dolomitic mudstones deposited in playalike supratidal flats. The upper 10–15 m of the Manlius Formation comprises meter‐thick cycles of nodular‐bedded, stromatoporoid‐bearing thin sets that grade up into microbial‐laminated mudstones. Much of this succession of rock types is traceable across nearly the entire basin and strongly suggests that lower beds of the Manlius Formation do not pass laterally westward into, nor are they coeval with, the underlying Rondout Formation. This, in turn, necessitates that the time‐transgressive model of Manlius and Coeymans limestone deposition proposed by Rickard is incorrect and that the Helderberg succession of Manlius–Coeymans–Kalkberg–New Scotland formations instead comprises one third‐order stratigraphic sequence. The lower sequence boundary may comprise the disrupted dolomitic mudstones overlying the microbial‐laminated mudstones in the middle of the Manlius Formation, an interval that appears to correlate with a disconformity between the top of the Manlius Formation and the Coeymans Formation in the section exposed at Schoharie, New York. This interpretation of stratigraphic relations within the Manlius Formation necessitates reevaluation of possible correlations among meter‐thick shallowing‐upward successions in the formation.

Transgressive wave ravinement on an epicontinental shelf as recorded by an Upper Pennsylvanian soil-nodule conglomerate-sandstone unit, Kansas and Oklahoma, U.S.A.

A thin (3–25 cm), persistent conglomerate-sandstone unit occurs at the base of the transgressive systems tract of an Upper Pennsylvanian cyclothem below a transgressive limestone in outcrop and subsurface in SE Kansas and NE Oklahoma. It has an erosional base and a sharp top, and is composed of calcitic clasts, some shell fragments and quartz sand, and rare coal fragments. Clasts are rounded, equant-to-elongate, coarse sand-to-pebble sized, and moderately to well sorted. They are micritic to microsparitic or radial fibrous calcite grains and pisoids. Micritic and microsparitic clasts are heterogeneous and contain rounded to elongate moulds and radiating or concentric, spar or micrite-filled cracks. Pisoids have micrite cores and superficial calcite or clay rims. The clasts have similar texture and composition to the calcitic nodules and rhizoconcretions in underlying paleosols, and thus were probably derived from these soil nodules. Upper shoreface erosion during shoreline transgression excavated the underlying paleosols and eroded coeval transgressive deposits landwards of the shoreline. The eroded sediments were then reworked and transported to the lower shoreface and inner shelf by storm return flows to be deposited as a soil-nodule conglomerate-sandstone unit. Its base is an amalgamated surface of wave ravinement and initial transgression as well as the lower sequence boundary. The widespread conglomerate-sandstone unit suggests extensive transgressive wave ravinement on an epicontinental shelf. The transgressive record composed of upward-deepening, dominantly fine-grained lithofacies defines a new type (T-C 4) of transgressive record, which is composed of mixed carbonate and siliciclastic lithofacies with a simple transgressive lag on a low-relief shelf.

Sequence response to syndepositional regional uplift: insights from high-resolution sequence stratigraphy of late Early Pleistocene strata, Periadriatic Basin, central Italy

This paper deals with the depositional sequences that occur within the uppermost part of the Plio–Pleistocene Periadriatic basin fill in the southern Marche region, central Italy. The succession is an Early Pleistocene, easterly dipping clastic wedge showing an overall shallowing trend from slope clays to shallow-marine and non-marine deposits comprising two major sequences, namely Q mb and Q mc. Analysis has provided new insights into: (i) the nature of sedimentary facies and facies associations occurring within the upper part of Q mb and Q mc; (ii) the gradual contact within Q mb between regressive littoral deposits (RLD) and underlying deep-marine blue clays; (iii) the composite origin of the Emilian surface, which is a widespread erosional unconformity separating Q mb from Q mc; (iv) the cyclothemic pattern of Q mc, composed of downstepping, small-scale depositional sequences; (v) the role played by synsedimentary uplift on the stacking pattern of small-scale sequences and their internal architecture. Up to three small-scale depositional sequences have been recognised within Q mc (Q mc1, Q mc2 and Q mc3). They are up to 50 m thick and defined by previously unrecorded, lower-rank, regionally extensive surfaces. Facies organization indicates that, as a rule, in ascending stratigraphic order each small-scale sequence may includes a distinctive basal unconformity surface that can be traced from incised valleys to associated interfluves, an incised-valley fill, a transgressive surface of marine erosion, a transgressive systems tract, a highstand systems tract, a regressive surface of marine erosion and an attached falling stage systems tract. In proximal positions, the small-scale sequences are dominated by transgressive and highstand systems tracts with incised-valley fills and falling stage systems tracts absent or volumetrically much less significant. In relatively basinward locations, where the regressive surfaces of marine erosion converge with the lower sequence boundary, the falling stage systems tract may represent the entire depositional sequence. High-frequency sequences display a distinctive stacking pattern and form a tectonically induced forced regressive sequence set underlain by a composite, tectonically enhanced regressive surface of marine erosion formed by the lateral connection of lower-rank sequence boundaries.

Evolution of the Glaciated Shelf and Coastline of the Northern Gulf of Maine, USA

The Gulf of Maine (northeast US coast) records shelf evolution since deglaciation ca. 15 ka. Glacial erosion of bedrock left a complex coastline of bays, peninsulas and islands. Till and outwash provided coarse sediment for reworking in littoral systems throughout the Holocene transgression. Glaciomarine mud, the Presumpscot Formation, was an abundant source of fine sediments that were reworked in estuaries, embayments and back-barrier systems. Relative sea-level change was driven by both isostatic and eustatic components. Initial submergence to 70–130 m above present sea level was contemporaneous with marine-based ice-sheet retreat at 15-13 ka. Rapid emergence followed to 60 m below present, during continuing isostatic rebound at 13-11 ka. Finally, submergence and transgression occurred 10.8 ka to present as isostatic rebound was overtaken by eustatic sea-level rise. Reworking during emergence and lowstand brought sand and gravel to the present coast and inner shelf, building paleodeltas at the mouths of the Merrimack River and Kennebec River. Other rivers, such as the Penobscot, drained landscapes with fewer coarse-grained sources, and have primarily mud-filled estuaries, such as Penobscot Bay.Detailed seismic reflection profiling and sidescan sonar mapping provide data for a model of inner-shelf evolution based on principles of sequence stratigraphy. The lower sequence boundary is the unconformity created on top of the Presumpscot Fm. and other glacial sediments during emergence. Lowstand systems tracts are best recognized in paleodeltas. Transgressive systems tracts are thin, but interrupted by parasequences of prograding deltaic and estuarine facies in some estuaries. Significant examples such as the newly discovered Penobscot Paleodelta (8–9 ka, 30 m below present sea-level) may relate to a slowing of relative sea-level rise. Highstand systems tracts formed in the late Holocene as the rate of sea-level rise slowed and the rate of sediment supply allowed stabilization and progradation of barriers and tidal deltas. Preservation potential of these features is controlled by the open coast ravinement unconformity, and by the tidal ravinement unconformity and bluff ravinement unconformity in embayments. Variability in preservation potential results both from paleotopography and differing energy of modern processes.

Hiatal duration of correlative sequence boundaries from Oligocene–Pliocene mixed carbonate/siliciclastic sediments of the south Florida Platform

Abstract Sequence boundaries defined by regional unconformities in mixed carbonate/siliciclastic sediments have a variable duration from the margin to the interior of the Florida Platform. Factors controlling the timing and duration of these unconformities include a combination of global (eustatic) and regional mechanisms (sediment supply, current intensification). Integrated chronostratigraphies developed from a transect of continuous cores in the south-central interior to the southern margin of the Florida Platform show an increase in duration of the hiatal gaps along correlative sequence boundaries toward the platform edge. Two sequence boundaries are correlated across the south Florida Platform and mark formation contacts. The lower sequence boundary (SB-A) marks the Suwannee Limestone/Arcadia Formation contact and is of significantly less duration in the platform interior (southwest peninsular Florida, ∼2.0 m.y.) than at the south platform margin (middle Florida Keys, ∼8.1 m.y.). The hiatal gap along the upper sequence boundary (SB-B), marking the Arcadia Formation overlain by Miocene–Pliocene sediments that vary in age and composition regionally, records the same increase from the central part of the platform (southwest Florida, Arcadia Fm/Peace River Fm, ∼1.6 m.y.) to the south platform edge (lower Florida Keys, Arcadia Fm/Stock Island Fm, ∼11.5 m.y.). Eustatic fluctuations affected the relative sedimentation rates of the mixed sediments in the platform interior and at the margin. Most notably, the large eustatic fall at the end of the Early Oligocene terminated carbonate deposition of the Suwannee Limestone (SB-A). Sediment accumulation on the platform interior was more temporally continuous than at the margin because of greater siliciclastic input from the north. The strengthening of marine currents beginning in the Middle Miocene limited sediment accumulation across the southern platform margin (SB-B) and possibly caused erosion of the margin carbonates, increasing the duration of the hiatal gap on top of the Arcadia Formation.

Sequence Stratigraphy of a Coastal-Plain Incised Valley Estuary: Lake Calcasieu, Louisiana

ABSTRACT The stratigraphic profile along a 100 km section of the Lake Calcasieu incised-valley fill was reconstructed from 87 bridge borehole records at four sites along the tidal reaches of the Calcasieu River, an additional 18 boreholes aligned along the chenier plain near the estuary mouth, and shallow seismic data collected along 160 km of track line within the Lake Calcasieu central basin. Valley incision occurred during the d18O stage 2 sea-level lowstand, when the paleo-Calcasieu River flowed across the inner continental shelf and incised into older fluvial terraces of the Prairie Formation. The valley floor is 57 m deep beneath the modern chenier plain and rises to -30 m along the upper reaches of the modern estuary. Sea.level rise commenced about 20 ka and has continued episodically throughout the Holocene. Shoreline transgression has proceeded at varying rates in response to sea-level rise, and the incised valley is now occupied by a wave-dominated estuary, fringed by the chenier plain of western Louisiana. Lowstand deposits are poorly preserved within the Calcasieu incised valley. Deposits constituting the transgressive systems tract (TST) lie directly above the lower sequence boundary at the top of the Prairie Formation. The TST shows greatest facies diversity and clearest separation of bounding surfaces toward the present seaward end of Lake Calcasieu. Here, the TST comprises three parasequences that include fluvial, bayhead-delta, and central.basin (estuarine) deposits. Multiple flooding surfaces within the TST relate to the episodic style of sea-level rise. Highstand infilling of the valley has been characterized by nearshore marine deposition and chenier-plain progradation. Along the lower reaches of the modern Calcasieu River, the valley fill comprises transgressive central-basin nd landward-stepping bayhead-delta facies, overlain by the highstand bayhead-delta complex. This study documents the most recent phase of deposition within the compound fill of a coastal-plain incised valley and demonstrates the potential importance of fluvial controls on facies within incised-valley systems.

Late quaternary sediments in incised coastal valleys in Benin: A preliminary sequence-stratigraphic interpretation

Abstract The incised coastal valley fill of the Mono and Oueme Rivers in Benin comprises several Late Pleistocene and Holocene facies that have been interpreted in sequence-stratigraphic terms. The study shows the existence of lowstand, transgressive and highstand systems tracts related to the last 120,000 year eustatic cycle and the subsequent still-stand since around 5500 BP. The lowstand systems tract consists of Late Pleistocene alluvial sediments overlying an earlier sequence in the Oueme valley. This tract has not been identified in the Mono Valley mouth where the lower sequence boundary directly separates the transgressive systems tract from the underlying substratum. The transgressive systems tract is represented by lagoonal mud and bay-mouth shoal/flood-tidal delta sands. This tract may also subsist as a thin shoreface lag or overwash sandy gravel deposit. The lagoonal muds are separated in places from the overlying estuary-mouth sands by tidal or wave ravinement surfaces. The highstand systems tract comprises the following: prograded sand barrier beach/shoreface sands, a non-prograding barrier associated with a migrating inlet, lagoonal muds and bay-head delta sands and muds. This tract has been partly reworked by tidally-influenced migration of the Mono channel and includes highstand fluvial-tidal, tidal and wave ravinement surfaces. Fine-scale differentiation of early highstand and late transgressive systems tracts and the maximum flooding surface, especially in the Oueme Valley, is rendered difficult by the essentially similar nature of their facies. This is notably the case of muddy backbarrier lagoonal sediments. It is hoped that future work will enable a finer resolution of these sequence tracts and stratigraphic surfaces.

温帯浅海域の炭酸塩堆積システム オーストラリア，ビクトリア州南部，第三系上部漸新統・下部中新統

With the purpose to envisage depositional systems of the temperate shelf carbonates with respect to sea level changes in a tectonically inactive region, eight sedimentary facies (facies A to H) are differentiated in a sedimentary sequence which occurs within the Jan Juc Formation (Late Oligocene to Early Miocene) along the sea cliff at Torquay area, southern Australia. The sequence ranges from 1 to 6m in thickness and over 3.5 m.y. in duration and their spatio-temporal relationship is restored.While the lower sequence boundary (25.5Ma) formed during a lowstand period varies basinward from karstified surface into a conformable surface within the heavily burrowed beds, the upper sequence boundary (22Ma) is traced along just below the well-indurated “Bird Rock Cap (BRC)”, which was deposited during the subsequent lowstand period.The sequence is unevenly divided into two packages of sediments by MFS (maximum flooding surface), which is traced within the interval less than 1m above the shell bed (facies E); the thick TST (transgressive systems tract) below MFS and the less developed HST (highstand systems tract) above it. The TST sediments of especially very fine packstone-mudstone of which fossils are largely made up of benthic foraminiferal tests and shell fragments (facies C) are best developed, onlapping onto the lower sequence boundary. The transgressive lag characterized with profusion of glauconitic minerals and shell fragments of gravel size (facies A) occurs basinward just above the sequence boundary. Sediments of the HST are mainly composed of fine packstone with bivalve fragments and benthic foraminiferal tests (facies F) and relatively thin as progradation took place only landward.It is concluded, therefore, that the most salient characteristic of the temperate carbonate depositional systems for a complete cycle of sea level change is low influx of sediments at early transgression and highstand periods, as evidenced by grain composition, especially abundance of glauconitic minerals, and difference in thickness between the TST and HST.

Estuarine, barrier-island to strand-plain sequence and related ravinement surface developed during the last interglacial in the Paleo-Tokyo Bay, Japan

Recognition of sequence boundaries and transgressive surfaces (i.e. ravinement surfaces, RS) is now known to be of great importance in stratigraphy. The sedimentary features of deposits immediately above a transgressive surface are well exposed in the Upper Pleistocene Kioroshi Formation of the Kanto Plain in central Japan. The formation comprises mainly coastal and shallow-marine deposits (estuarine, barrier-island and the strand-plain systems) which accumulated along a wavedominated coast in the Late Pleistocene, i.e., the last interglacial to last glacial period. The Kioroshi Formation is bounded above and below by sequence boundaries that formed in the lowstand periods correlative to the glacial periods of oxygen isotope stages 4 and 6, respectively. A significant transgressive surface that was formed by landward migration of barrier islands during the transgressive interval, the ravinement surface (RS), is found within the deposits of the upper shelf environment. This ravinement surface is characterized by the exotic nature of the overlying sediment veneer (pebbles, shells and scattered mud clasts) which is poorly sorted. The RS shows a very flattened erosional surface in the shore-parallel sense, and the gradient of the surface in shore-normal sense is calculated as 0.0021, where the syndepositional tectonic movement is revised. The RS commonly cuts through the lower sequence boundary. However, in the places where the river or tidal channel valleys incised, the valley-filling sediment shows a deepening-upward sequence recognized as a transgressive systems tract and the RS can be clearly distinguished from the lower sequence boundary.

Biostratigraphic, Chronostratigraphic, and Stratigraphic Sequence Analysis of Lower Tertiary Marine Sediments of Alabama for Indicators of Sea-Level Change

Early Eocene to late Oligocene marine sedimentary units in southwestern Alabama were sampled at closely spaced intervals to derive a precise time-stratigraphic framework and to determine the paleoecological and mineralogical responses to fluctuations in sea level. Paleontologic control consisted of planktonic, smaller and larger benthonic foraminifera, calcareous nannofossils, dinoflagellates, and megafossils. Paleomagnetic reversals were delineated in two boreholes which, when supplemented by strontium isotope dates and the biostratigraphic control, provided a robust in situ chronostratigraphy for the Gulf Coast lower Tertiary. Paleoecologic trends in regression and transgression can be clearly correlated across major regional facies changes. Using the chronostratigraphy developed here, the second-, third-, and fourth-orders of Vail's global sea-level cycles can be recognized and demonstrate the influence of sea-level change on sedimentation. Stratigraphic systems tracts (SSTs) and bounding surfaces in outcrop were determined by lithologic variations and paleoecologic trends, and additionally by gamma logs in the cores. The lower sequence boundary occurs at a contact where an older, relatively fine-grained, deep-water, fossiliferous unit was abruptly succeeded by a coarse-grained, shallow-water, poorly fossiliferous unit. The transgressive surface occurs at the base of a fining- and deepening-upwards unit that was commonly glauconitic and very fossiliferous. Transgression culminated with a pulse ofmore » planktonic microfossils in a bed having reduced clastic sedimentation; on the log the surface of maximum starvation was marked by a gamma spike.« less

Depositional Setting of the Jurassic Haynesville Seismic Sequence in the Apalachicola Basin, Northeastern Gulf of Mexico: ABSTRACT

Seismic and well data from the northeastern Gulf of Mexico were used to define the seismic stratigraphy, geologic history, and depositional setting of the Jurassic (Kimmeridgian) Haynesville sequence in the Apalachicola basin. The data show that Haynesville clastic sedimentation updip was coeval with Haynesville carbonate deposition downdip. The regional Jurassic seismic stratigraphic framework includes, in ascending order, the Louann Salt Norphlet-Smackover, Haynesville, and Cotton Valley sequences. In the vicinity of Destin dome, wells have penetrated Haynesville sandstones, shales, and anhydrites. These clastics correlate with low amplitude, low-continuity reflections that characterize the Haynesville over a broad area updip. Similar reflections within the overlying (Tithonian-earliest Berriasian) Cotton Valley clastic sequence make seismic definition of the top Haynesville sequence boundary difficult updip. As Haynesville clastics are replaced by carbonates downdip, a high amplitude reflection marks the top of the sequence. Haynesville carbonates conformably overlie (Oxfordian) Smackover carbonates in the basin center, and the lower sequence boundary cannot be defined where disrupted by growth faults associated with early movement of the (Callovian ) Louann Salt. Sigmoid clinoforms document Haynesville shelf margin development Seismic facies also include oblique clinoforms that prograde eastward into the basin from the Southern Platform and Middle Ground Arch. No wellsmore » penetrate this facies. Mapping of the seismic facies and correlation with well data suggest a depositional setting for the Haynesville sequence in which influx of terrigenous clastics probably derived from adjacent land areas to the north and northeast filled a broad lagoon behind a carbonate shelf margin.« less